Initially, sintered products produced from zircon and zirconia were essentially used as hearth sublayers for glass furnaces because their corrosion resistance was insufficient to be used directly in contact with glass.
Increasingly, these products are tending to be used in contact with glass; they have in fact a lower corrosion resistance than products based on chromium oxide, but they do not present a risk of colouring the glass.
U.S. Pat. No. 5,124,287, assigned to Corhart Refractories Corporation, describes dense zircon-based products that are improved as they are resistant to thermal shock and can be more easily used in contact with glass. These products essentially consist of zircon and of small additions of zirconia and titanium oxide. The authors mention contents of zirconia ranging from 5 to 25% introduced into the composition. The examples show that, for products containing more than 25% zirconia, cracks appear when firing the blocks and this even occurs in the case of small blocks having a mass of less than 10 kg. This patent mentions that if other compounds are present they must preferably represent less than 2% by weight if it desired to maintain a corrosion resistance level identical to that of products essentially consisting of dense zircon. The authors also point out that excessively high percentages of zirconia would result in high costs and a tendency to cause stoning. Moreover, it is mentioned that it is preferable to use monoclinic zirconia, and therefore to avoid the presence of stabilizers such as yttrium oxide.
Corhart currently sells a product of the type described in the abovementioned patent under the name ZS-1300; it is this product that is mostly used in bottoms of bottom-electrode furnaces.
Other products produced from zircon and zirconia have been described in the literature. Many of these are intended to be used in contact with molten metal. For this application, the constraints are different from those encountered in glassmaking. This is because, in metallurgy, the temperatures at which refractory blocks are used are different from those encountered in glassmaking. In addition, in metallurgy it is possible to use, without any particular problem, zirconia stabilizers such as MgO or CaO. However, in glassmaking, these stabilizers are unsuitable when the refractory blocks are in contact with any vapour emitted by the glass (as in the case of glass feeders). This is because such vapour damages the refractory block and cracks form. This may result in the product crumbling and therefore causing stoning in the glass, this being synonymous with defects. However, any defect in the glass, particularly in glass for reinforcing fibres, in unacceptable.
In glassmaking, there is presently a need for a more resistant material, particularly for electrode-supporting blocks in bottoms and in feeders for furnaces making glass for reinforcing fibres. These materials, in addition to improved corrosion resistance, must satisfy other criteria. First of all, their industrial feasibility must be guaranteed. "Industrial feasibility" should be understood to mean the possibility of obtaining large crack-free blocks since, in the construction of glass furnaces, the blocks are of large size; yet, the greater the size of the pieces, the higher the stresses and therefore the higher the risk of cracking after firing. Thus, the possibility of producing crack-free blocks with a mass of greater than 10 kg will be chosen as feasibility criterion. In fact, it is important to avoid the presence of cracks which would be preferred sites of corrosion. For the same reason, these products must have as low a porosity as possible. Moreover, given the low viscosity of glass for reinforcing fibres, it is necessary to ensure that the joints between blocks are sealed at the operating temperature of the furnace. Finally, it is essential to limit the costs.
The invention aims specifically to meet this need.
While seeking to obtain a product which meets the various requirements mentioned above, we have demonstrated that it is possible to obtain a more corrosion-resistant material. Unlike the teachings of U.S. Pat. No. 5,124,287, it has been found that the presence of relatively high percentages of zirconia does not result in a particular tendency to cause stoning and that the incorporation of various additives allows the various requirements mentioned above to be met.